High strength steel and method of treatment thereof



United States Patent 3,151,979 HG STRENGTH STEEL AND lt EETHOD 10F TREATMENT THEREGF Dennis J. Carney, Samuel J. Manganeiin, and John E.

Steiner, Pittsburgh, Pa, assignors to United States Steel Corporation, a corporation of New Hersey N0 Drawing. Filed Mar. 21, 1952, Ser. No. 181,434 12 Claims. (Cl. 75-128) This invention relates generally to high strength nonmagnetic steels and their treatment, and more particularly to steels for generator retaining rings and a method of treating the rings to produce a high yield strength.

This application is a continuation-in-part of our copending application, Serial No. 106,452, filed May 1, 15 1961, now abandoned.

In the electric power industry, there is need for highstrength, ductile, fine-grained, nonmagnetic steel ring forgings for coil supports in generator rotors. These ring forgings are usually referred to as retaining rings. These rings conventionally range in size from 27 to 6 3 inches in outside diameter, 19 to 30 inches in length, and 2 to 6 inches in Wall thickness. These retaining rings must Withstand high operating stresses, and must be nonmagnetic (less than about 1.2 magnetic permeability) to prevent current losses.

As the size and capacity of generators increase, higher Other objects and a fuller understanding of this invention may be had from the following description and claims.

According to this invention, suitable combinations of solution treatment, cold working, and aging are applied to austenitic steels of a given composition to produce retaining rings that exhibit high yield strength and good tensile ductility required of these retaining rings. The broad composition ranges for such a steel are as follows:

wherein X is Cb or a mixture of Cb and Ta. Table II below indicates 13 specific compositions of steel which have been used to evaluate retaining-ring steels having the requisite high yield strength and tensile ductility required.

Table 11 Steel A B C D E F G H I I K L M 2. 7 V. 0.80 0. 85 0. 82 0.02 0.02 0. 16 0.25 0.39 Al 1 0. 010 0. 005 0. 000 0. 004 0. 009 0. 004 0.007 0. 007 0. 009 0 007 0. 009 0. 008 0. 008 N 0. 44 0.44 0. 39 0. 26 0. 39 0.34 0.39 0. 38 0.36 0. 42 0.35 0. 33 0. 34 01 0.39 0.17 0.27 0.18 0.18 0.18 Fe and residnal impurities Bal. Bal. Bal. Bal. Bal. Bal Bal. Bal. Bal. Bal. Bal Bal. Bal.

Acid soluble.

Norm-Blanks denote residual amounts (less than 0.05%) of these elements.

strength retaining rings are required. The strength requirements of retaining rings for generators now being designed is about 170,000 p.s.i. yield strength at .2 percent ofiset and an elongation of approximately 15 percent. This compares with prior art retaining rings which need a strength of only about 125,000 p.s.i. yield strength at 0.2 percent Oi set and an elongation of approximately 20 percent.

It is therefore a principal object of this invention to provide a nonmagnetic steel and method of treatment thereof to produce a high yield strength.

It is a more particular object of this invention to provide a steel suitable for forming generator retaining rings and a method of treating the rings formed therefiom to provide high yield strength.

conventionally, steel retaining rings for generators are formed by providing an ingot and forging the ingot at about 2200 F. to a blank, punching a hole in the forged blank, mandrel forging to enlarge the hole and Water quenching. Rough machining may be performed at this point and the blank ultrasonically inspected if the grain size is small enough to so permit. The forged blank is then solution treated at temperatures between 1900 F. and 2200 F. and Water quenched. The blank is then expanded at temperatures below 450 F., and a final machining may be performed. A final ultrasonic inspection may then be performed if the grain size is sufiiciently small. One method of expanding the blank is described in application Serial No. 37,719, filed June 21,

. 3 4 1960, entitled .Method and Apparatus for Enlarging certain of the Steels tested to perform the expanding in Rings. two steps and age the steel after each step.

It has been found that if the rings are aged at tern- Table III below indicates various treatments given to peratures between 500 F. and 1400 F. after they have test specimen bars formed from various compositions of been expanded, a substantial increase in yield strength 5 steel listed in Table II above and test results of such is obtained. Also, it has been found to be beneficial for treatments.

Table 111 Y- 3 95st 16 Solution Amount of Aging Strength Tensile Elongation Reduction Test, Steel Temp., Prestrain, Temp., (0.2% Strength, 1n21nches, of Area, Energy F. Percent F. onset), p.s.i. Percent Percent Absorbed p.s.i. at Room Temp., 11.15.

35 103, 500 173,800 28. 0 A 1,950 35 000 175,000 180,700 20. 0 238 1 8288 2% 0 108, 7 950 35 000 181,000 130,800 20.5 B 4 197,100 202,3 gn 0 170,400 7 c 1, 950 35 000 184,900 189 200 23. 00 2 288 382988 it? 5 1 90 000 191,900 190,300 20.5 C 0 35 i 700 181,400 500 187,200 000 184, 900 189, 200 23. 5 O 1950 35 700 182,900 182,3 8 30 100,400 7 00 35 000 182,200 187,200 23. 5 C l 1382 312 0 109, 00 1); 1,950 3 000 177,000 184,900 22.5 a 40 82 13 38 32 30 16 1) 000 171,200 180,300 24.0

2000 28 180,500 192,100 22.0 3 .0 000 109, 900 178, 500 25. 5 51. 7 25 700 180, 800 184, 700 24. 0 51.0 1,950 30 000 192,500 190 000 18. 0 48.5 000 210,400 213, 000 1 7. 0 43. 0 1, 300 103, 100 173, 300 24. 0 45. 0 20 1, 050 185,000 191, 500 23. 5 40. 0 F 2,100 30 1,175 178,900 130,200 23.5 39.5 1, 300 178, 000 189,400 18. 5 23. 0 4 925 170, 200 181. 000 20. 0 41.5 000 25 2 1, 050 170,200 183,000 21. 0 41.5 G 2 925 170,100 182, 000 22. 0 48. 0 050 25 2 1, 050 174, 100 183, 100 23. 0 45. 5 925 184,000 189,300 22.0 50.1 4 923 182, 50 127, 2 3: 1. 2 1, 05 170,10 1 5, .5 G 2,100 25 4 1, 050 188, 300 190,900 21. 5 39. 5

1,175 185,700 194 900 19.0 37.5 1,175 181,300 192,900 13.5 30.5 800 187,800 191 200 15.5 29.8 800 205,100 205,100 17.5 40.9 5 3.23 1333 32 G 2 1 1 i 30 925 190,000 199,700 20.0 41.7 1,050 195,100 200,500 20.0 43.0 1,050 193,000 190,000 22.0 39.8 925 100,900 175,400 21.0 43.3 4 92 5 172, g 178, 2g. 1,05 109,20 177 2 4 11 25 4 1, 050 171,700 178,400 23.5 33. 3 1,175 104,800 177.100 24.5 41. 5 4 1,175 108, 400 180,200 21.5 32. 3 800 194, 200 197, 300 17. 0 41. 9 4 300 199, 400 199,400 19. 5 41. 3 4 900 181,800 184, 18. 5 33. 5 H 2, 100 30 925 194,500 200, 100 18.5 42.2 2 925 195,700 197,000 19. 5 42. 1 1, 050 189,900 190.400 19.5 40. 5 4 1, 050 194,700 197, 500 19. 5 37. 5 H 2, 100 33 5 900 213,200 213,200 17. 5 40.0 25 700 170, 400 175, 700 25. 0 53. 0 I 1, 950 4 20 700 108,700 171,900 20. 0 50. 0 4 32 700 181, 700 181,700 21. 0 48. 8 25 700 107,700 171,700 24.0 52. 7 J- 1, 950 4 25 700 100,800 109,500 24.0 50. 0 4 30 700 175, 700 175, 700 22. 0 51. 5 25 700 171,900 179,500 24. 0 53.0 K 1, 950 4 25 700 100. 400 170, 000 20. 0 50.9 4 31 700 173,500 174, 000 22. 0 53. 0 25 700 105, 100 170,400 20. 0 50. 5 L 1,950 4 25 700 102,900 107,500 27. 0 53.0 4 30 700 170,700 171,200 25. 0 52. 1 M 1,950 4 31 700 179,500 179, 500 10. 0 47. 0

1 Broke at gage mark.

2 Aged for 7 hours and air-cooled to room temperature. 3 Aged for 4 hours and air-co0led to room temperature. 4 Prestrained at 350 F.

Norm-All specimen blanks (1% inches square by 6 inches long) were solution-treated for 2 hours at the indicated temperatures, except specimens 1'. through M which were solution-treated for 4 hours at the indicated temperatures, air-cooled to room temperature (to simulate the water quenching of retaining rings with a wall thickness of about 4 inches), machined to 0.600dnch-diameter tensile specimens, prestrained (elongated) the indicated amounts, aged at the indicated temperatures for 2 hours (except where noted), air-cooled to room temperature, then pulled to failure. All fractures were nonmagnetic (below 1.2 Permeability).

5 Table IV below shows the treatment and results for steels G and H, wherein the specimen bars were strained in two steps and aged after each step.

The austenitic grain sizes of each of these steels solution treated at various temperatures are given in Table V below:

Table IV Yield Solution Amount of Aging Amount of Aging Strength Tensile Elongation Reduction Steel Temp., Prestrain, Temp., Prestrain, Temp., (0.2% Strength 1n2inches, of Area, F. Percent F} Percent F? Ofiset), p.s.1 Percent Percent 25 1, 050 188,300 190, 900 21. 5 s9. 5 1, 050 25 1,050 166, 100 175,900 29. 46. 4 10 1, 050 15 1, 050 177, 200 183, 600 26. 0 47. G 2 100 15 1,050 1,050 176,000 182,200 25.0 40.0 1 25 925 182, 500 187,700 20. 0 39.2 1, 050 25 925 177, 100 184, 000 21. 5 45. 0 10 925 925 183, 000 186, 200 23. 5 46. 0 10 1, 050 15 925 168, 800 176, 600 22. 0 47. 0 1, 050 171, 700 178, 400 23. 5 33. 3 1, 050 25 1, 050 175, 400 184, 800 24. 0 46. 4 10 1, 050 15 1, 050 166, 490 175,900 29. 0 45. 2 H 2 100 15 1,050 10 1,050 163, 400 173,300 26.0 40.0 1 925 172,400 178,700 22.5 45.0 1, 050 25 925 174, 400 181, 500 22. 5 46. 0 10 925 15 925 174,400 150, 400 22. 5 42. 0 10 1,050 15 925 176, 300 180, 800 21. 5 42. 0

1 Aged for 2 hours and air-cooled to room temperature. 2 Aged for 7 hours and air-cooled to room temperature.

The above results indicate that the steel should be solu- Table V tion treated at temperatures between 1900 F. and 2200" F., the strain should be between 15 and 40 percent, and N S Austenite Grain Size, ASTM 0., of Steels olutionthe agmg. should be between 500 F and 1400 F Treated for 2 Hours at Indicated Temperature More particularly, steels A, B, C, D, E, I, J, K, L and M s m 0 preferably should be solution treated between 1900 F. F. 1950, F' 21000 F. and 2050 F., strained between 20 percent and 40 percent and aged between 500 F. and 800 F. Preferably steels 2 to 3 of the composition F, G and H should be solution treated 2% V 223% between 2000 F. and 2200 F., strained between 20 and 1%t0'3TIII 1 /5 :12:

1 1 1 40 percent and aged at temperatures between 800 F. m 4%) 5% and 1200 F. Preferably rings made of steels A, B, C 6t0 g totd fi 1 and D should be expanded at temperatures in the range 5m o 200 F. to 450 F. between 34 and 38 percent and aged g: between 500 F. and 800 F. with the prior solution 5to61:::: treatment being at between 1900 F. and 2050 F. Rings 5 t0 formed from steels E, I, I, K, L and M should be expanded at temperatures in the range F. to 450 F. between 24 to 32 percent and aged between 500 F. and 800 F. with the prior solution treatment being at between 1900 F. and 2100 F. Rings formed from steels F, G and H should be solution treated between 2000" F. and 2200 F and expanded at ambient tempeartures between 24 and 32 percent and aged between 800 F. and 1200 F.

Generally, the optimum temperature for solution treating steels A, B, C, D, E, I, J, K, L and M is about 1950 F., and for steels F, G and H about 2100 F. Increasing the solution temperature in general induces a decrease in yield and tensile strengths and an increase in tensile ductility as would be expected.

The optimum aging temperature is generally lowered for steels F, G and H as the amount of strain (cold work) of each steel increases. For steel C, strained to 35 percent, the yield and tensile strengths decreased and tensile ductility increased as the aging temperature was increased from 500 F. to 700 F. However, for steelE strained to 25 percent, the yield and tensile strength increased and the tensile ductility decreased as the aging temperature increased from 600 F. to 700 F. In steel D the austenitic grain size was moderately coarse, the effects of which will be discussed later. Steels E, G, H and K have the best properties in Char py V-notch tests. For steels according to this invention, the energy absorbed in a room temperature Charpy V-notch test should exceed 15 ft. pounds.

1 Some specimens of these steels exhibited duplex grains, some as large as ASTM No. 1 to 2.

Retaining rings having a large grain size (ASTM No. 2 or larger) are not amenable to ultrasonic testing. However, rings having a fine grain size (ASTM No. 4 or smaller) are amenable to ultrasonic testing. ASTM grain size No. 3 is a borderline case. Steels A through E and I through M solution treated at 1950" F. and steels F through H at 2100 F. are amenable to ultrasonic testing, although steels A and D would be borderline cases. The A coarse grain size also seriously impairs yield strength and impact strength of these steels.

Certain elements exert a specific effect on the properties of these steels. Increasing the silicon content from 0.46 percent (steel A) to 0.74 percent (steel B) promoted a moderate increase in yield and tensile strength and a moderate decrease in tensile ductility. Decreasing the chromium content from about 17.80 percent (steel C) to 13.10 percent (steel D) promoted the retention of only 0.26 percent nitrogen in steel D as compared to 0.39 percent nitrogen in steel C; since nitrogen increases the austenite stability of austenitic steels and contributes toward high yield strength, the carbon content was increased from 0.25 percent (steel C) to 0.41 percent (steel D) to maintain good austenite stability and high yield strength. The addition of 0.39 percent colurnbium (steel E) promoted a very fine austenite grain size (ASTM No. 6 /2 to 7 /2) as compared to that (ASTM No. 3 /2 to 4 /2) of steel B, a steel with a composition similar to steel E, except for the absence of columbium. The addition of columbium of cold work; in the absence of these elements (see the X An effective amount up to .25% to promote fine. austenitic grain size and an increase in yield and e tensile strength.

Iron and residual impurities Bal.

wherein X is selected from a group consisting of columbium and mixtures of columbium and tantalum, said steel data for steel C), greater amounts of cold Work would be necessary to achieve the desired high strengths. Reducing the molybdenum content from 2.07 percent (steel G) to residual, i.e., less than 0.05 percent (steel H) promoted a decrease in yield and tensile strength, and greater amounts of cold work were required to achieve the desired high strengths.

It has been found that in some cases larger amounts of columbium, e.g., 39%, may tend to segregate into undesirable, eutectic-like, semi-continuous columbium carbides during solidification of heavy ingots. This segregation is particularly noticeable in the upper half of the ingots. Therefore it has been found that a lower amount of columbium tends to reduce either the tendency of the columbium to segregate or the harmful effects of the segregation or both without substantially impairing the physical properties of the steel. Although the lower columbium content has some slight effect toward coarser grains, cf. steel E with steels I through M, this efiect is no serious and the grain size is sufficiently fine to permit ultrasonic inspection. It has further been found that when the columbium content is lowered, as in steels I through M, the addition of vanadium has some beneficial effect. It is believed that *the vanadium does not tend to segregate to the extent'thatcolumbium segregates and thus will tend to promote uniformly fine grain throughout the ingot structure. 7 7

While We have shown and described certain preferred embodiments of the invention, it is' apparent that other modifications may arise. Therefore, we'do not wish to be limited to the disclosure set forth butonly by the scope of the appended claims.

We claim: a a r r V 1. An austenitic stainless steel, in the strained and aged condition, consisting essentially of:

Carbon .10 to .30%.

Manganese 14 to 17%.

Silicon .45 to .85%.

Nickel .05 to 1.0%.

Chromium 13 to 20%.

Nitrogen .20 to .45%.

X An effective amount up to .40% to promote fine austenitic grain size and an increase in yield and tensile strength. Iron and residual impurities- Bal.

wherein X is selected from a group consisting of columbium and mixtures of columbium and tantalum, said steel being characterized by a yield strength at 0.2% offset of at least 170,000 p.s.i.

2. An austenitic stainless steel, in the strained and aged condition, consisting essentially of:

.50% to promote an increase in yield and tensile strength.

being characterized by a yield strength at 0.2% offset of at least 170,000 psi.

3. In a method of manufacturing generator retaining rings from a steel having a composition as follows:

Percent Carbon .10 to .45 Nlanganese 12 to 20 Silicon .40 to 1.00 Nickel 8.0 max. Chromium 13 to 20 Molybdenum 2.5 max. Vanadium 1.0 max. Nitrogen .20 to .45 columbium .40 max. Iron and residual impurities Bal.

and wherein the steel is forged to a blank, a hole is punched in the blank and enlarged at forging temperatures, and the blank is Water quenched, and thereafter the blank is solution treated between 1900 F. and 2200 F., and thereafter the blank is expanded at least once, and wherein the total expansion is between 15% and 40%, the improvement which comprises aging the blank between 500 F. and 1400 F. after each time it is expanded, whereby to produce a yield strength, at .2% offset of at least 170,000 p.s.i.

4. In a method of manufacturing generator retaining and wherein the steel is forged to a blank, a hole is punched in the blank and enlarged at forging temperatures, and the blank is water quenched, and thereafter the blank is solution treated between 1900 F. and 2050 F., and thereafter the blank is expanded and wherein the total expansion is between 34% and 38%, the improvement which comprises aging the blank between 500 F. and 800 F. after it is expanded, whereby to produce a yield strength at .2% offset of at least 170,000 psi.

5. In a method of manufacturing generator retaining rings from a steel having a composition as follows:

Carbon .10 to 30%.

Manganese 14 to 17%.

Silicon .45 to .85%.

Nickel .05 to 1.0%.

Chromium 16 to 20.00%.

Molybdenum 0.10% max.

Vanadium .05% max.

Nitrogen .20 to .45%

X An effective amount up to .40% to promote fine austenitic grain size and an increase in yield and tensile strength. Iron and residual impurities- Bal.

wherein X is selected from a group consisting of columbium and mixtures of columbium and tantalum, and

wherein the steel is forged to a blank, a hole is punched in the blank and enlarged at forging temperatures, and the blank is water quenched, and thereafter the blank is solution treated between 1900 F. and 2050 F., and thereafter the blank is expanded and wherein the total expansion is between 24% and 32%, the improvement which comprises aging the blank between 500 F. and 800 F. after it is expanded, whereby to produce a yield strength at .2% ofiset of at least 170,000 p.s.i.

6. In a method of manufacturing generator retaining rings from a steel having a composition as follows:

.50% to promote an increase in yield and tensile strength.

Nitrogen .20 to .45%. X An effective amount up to .25% to promote fine austenitic grain size and an increase in yield and tensile strength.

Iron and residual impurities Bal.

wherein X is selected from a group consisting of columbium and mixtures of columbium and tantalum, and wherein the steel is forged to a blank, a hole is punched in the blank and enlarged at forging temperatures, and the blank is water quenched, and thereafter the blank is solution treated between 1900 F. and 2050 F., and thereafter the blank is expanded and wherein the total expansion is between 24% and 32%, the improvement which comprises aging the blank between 500 F. and 800 F. after it is expanded, whereby to produce a yield strength at .2% offset of at least 170,000 p.s.i.

7. In a method of manufacturing generator retaining rings from a steel having a composition as follows:

Percent Carbon .15 to .30 Nlanoanese 14 to 17 Silicon .45 to .85 Nickel 4.5 to 6.0 Chromium 16 to 20.00 Molybdenum 1.5 to 2.5 Vanadium .70 to .90 Nitrogen .20 to .45 Iron and residual impurities Bal.

rings from a steel having a composition as follows:

Percent Carbon .15 to .30 Manganese 14 to 17 Silicon .45 to .85 Nickel 4.5 to 6.0 Chromium 16 to 20.00 Molybdenum 1.5 to 2.5 Vanadium .70 to .90 Nitrogen .20 to .45

Iron and residual impurities Bal.

and wherein the steel is forged to a blank, a hole is punched in the blank and enlarged at forging temperatures, and the blank is water quenched, and thereafter the blank is solution-treated between 2000 F. and 2200 F., the improvement which comprises, expanding the blank twice, the total expansion being between 20% and 40%, and aging the blank between 800 F. and 1200 F. after each time it is expanded, whereby to produce a yield strength at .2% offset of at least 170,000 p.s.i.

9. A generator retaining ring comprising an annular steel body, the steel of said body consisting essentially of:

Percent Carbon .20 to .45 Manganese 14 to 17 Silicon .45 to .85 Nickel .05 to 1.0 Chromium 16 to 20.00 Molybdenum 0.10 max. Vanadium .05 max. Nitro en .20 to .45 Iron and residual impurities Bal.

said ring being characterized by -a yield strength at 0.2% offset of at least 170,000 p.s.i.

10. A generator retaining ring comprising an annular steel body, the steel of said body consisting essentially of:

said ring being characterized by a yield strength at 0.2% offset of at least 170,000 p.s.i.

11. A generator retaining ring comprising an annular steel body, the steel of said body consisting essentially of:

Carbon .10 to 30%.

Manganese 14 to 17%.

Silicon .45 to Nickel .05 to 1.0%.

Chromium 16 to 20.00%

Molybdenum 0.10% max.

Vanadium .05% max.

Nitrogen .20 to .45%.

X An effective amount up to .40% to promote fine aus tenitic grain size and an increase in yield and tensile strength.

Iron and residual impurities- Bal.

wherein X is selected from a group consisting of columbium and mixtures of columbium and tantalum, said ring being characterized by a yield strength at 0.2% offset of at least 170,000 p.s.i.

12. A generator retaining ring comprising an annular steel body, the steel of said body consisting essentially of:

Carbon .10 to 30%.

Manganese 14 to 17%. Silicon .45 to .85%.

Nickel .05 to 1.0%. Chromium 16 to 20.00%. Molybdenum 0.10% max.

11 Vanadium An effective amount up to .50% to promote an increase in yield and tensile strength. Nitrogen .20 to .45%. X An efiective amount up to wherein X is selected from a group consisting of columbium and mixtures of columbium and tantalum, said ring being characterized by a yield strength at 0.2% 15 oflset of at least 170,000 p.s.i.

r 1 12 References Cited in the file of this patent UNITED STATES PATENTS 2,711,959 Delong et al. June 28, 1955 2,789,048 Delong et al. Apr. 16, 1957 FOREIGN PATENTS 549,138 Belgium July 14, 1956 607,392 Canada Oct. 25, 1960 622,504 Canada June 20, 1961 OTHER REFERENCES A.S.T.M. Preprint No. 75, 1959, pages 1 to 6. Presented at the sixty-second annual meeting of the Society June 21 to 26, 1959.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3 151,979 October 6 1964 Dennis J Carney et a1,

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Columns 1 and 2 Table 11 column "J", line 10 thereof for "O 007" read 0.007 column 7., line 69 for "10%" read 1.0%

Signed and sealed this 26th day of January 1965.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Allesling Officer Commissioner of Patents 

12. A GENERATOR RETAINING RING COMPRISING AN ANNULAR STEEL BODY, THE STEEL OF SAID BODY CONSISTING ESSENTIALLY OF: 